With miniaturization of a semiconductor integrated circuit, a pattern formed on a semiconductor substrate, which is a base of the semiconductor integrated circuit, is miniaturized, and thus such a semiconductor substrate is required to have higher qualities. In particular, the requirement for flatness is particularly increased among various qualities required for semiconductor substrates. Among semiconductor substrates, an epitaxial wafer, which is suitable for various applications, needs to have both flatness of a substrate and flatness of an epitaxial layer. The flatness of the epitaxial layer significantly depends on film thickness distribution of the epitaxial layer. Therefore, in order to satisfy the required flatness of the epitaxial layer, it is necessary to improve uniformity of film thickness distribution of the epitaxial layer.
At present, a single-wafer-processing type vapor phase growth apparatus is used for manufacturing an epitaxial wafer having a diameter of 300 mm. Such a vapor phase growth apparatus is roughly composed of: a mechanism which supplies material gas for growing an epitaxial layer on a substrate; a reactor which grows an epitaxial layer on the substrate by using the supplied material gas; and a mechanism which discharges the gas from the reactor. The material gas supplying mechanism includes an injection cap (hereinafter referred to as “cap”), a baffle, and an injection insert (hereinafter referred to as “insert”) in order from the upstream side of the material gas. The cap has a space through which the material gas passes when the material gas is introduced into the reactor. The baffle is a plate-shaped member sandwiched between the cap and the insert, and has a plurality of through-holes which guide the material gas in the cap to the insert. The through-holes adjust the flow of the material gas toward the insert. The insert includes a plurality of flow paths which guide the material gas having passed through the through-holes of the baffle, to an inlet to the reactor. The material gas is guided to the reactor through these members. The reactor, to which the material gas is introduced, includes: an inlet which communicates with the reactor and into which the material gas from the upstream side flows; an outlet which is located above the inlet and closer to the reactor than the located of the inlet, and reaches inside the reactor; a passage connecting the inlet and the outlet; and a step portion located inside the passage. The material gas guided from the insert to the inlet of the reactor climbs over the step portion in the passage that reaches inside the reactor, and is guided into the reactor. The guided material gas is caused to react on the substrate, whereby an epitaxial layer is grown on the substrate. The gas generated through reaction of the material gas in the reactor and the unreacted material gas are discharged from the reactor by the gas discharging mechanism.
In order to grow an epitaxial layer whose film thickness distribution is made more uniform by using the single-wafer-processing-type vapor phase growth apparatus as described above, it is most important to guide uniform flow of the material gas onto the surface of the substrate in the reactor. In the present single-wafer-processing-type vapor phase growth apparatus, the flow of the material gas once introduced into the cap is changed to desired flow by the baffle, and then the material gas flows into the plurality of (e.g., ten) flow paths in the insert. However, the flow of the material gas itself, formed through the baffle is dominated by pressure balance in the cap, so that the speed corresponding to the diameter of each through-hole of the baffle cannot be obtained. Furthermore, since the flow of the material gas finely divided through the baffle passes through the insert to be guided onto the substrate, the flow of the material gas depends on the number of the flow paths in the insert. Therefore, variations in speed corresponding to the number (e.g., 10) of the flow paths of the insert are caused in the material gas that flows in the in-plane direction of the substrate, and speed distribution of the material gas introduced onto the substrate is determined based on the situation. Meanwhile, the material gas introduced into the inlet of the reactor climbs over the step portion in the passage communicating with the reactor, and is guided into the reactor. Thus, the flow of the material gas is influenced by the shape of the step portion. Specifically, the step portion located in the passage includes: a first surface which curves like an arc around an axis extending in the vertical direction on the reactor side, and opposes the inlet of the passage; and a second surface extending from an upper end of the first surface to the outlet of the passage. Therefore, the flow of the material gas introduced into this passage is biased outward in the width direction of the passage due to the first surface when the material gas is about to climb over the step portion. Accordingly, the speed distribution of the material gas, which has been controlled outside the reactor, is changed before the material gas is introduced into the reactor, which makes it difficult to minutely control the speed distribution of the material gas introduced onto the substrate. Because of the structural restriction on the vapor phase growth apparatus and the difficulty in controlling the speed distribution of the material gas, it is difficult to satisfy evenness of the film thickness distribution of the epitaxial layer needed in the epitaxial wafer used for advanced device.
In order to satisfy evenness of film thickness distribution, the shape of a top dome, which is one of components constituting a ceiling of the reactor, has been optimized. This optimization results in overall improvement of the film thickness distribution in the epitaxial layer grown on the substrate. However, the speed of the material gas, which is introduced through the insert in the in-plane direction of the substrate, still has a plurality of variations corresponding to the flow paths in the insert. If an epitaxial layer is grown by the material gas having such variations in speed on, for example, a substrate rotating around the axis extending in the vertical direction, the variations in the speed of the material gas causes concentric variations in the film thickness of the epitaxial layer. Since an epitaxial wafer having such variations in film thickness cannot satisfy the required flatness, it is necessary to reduce the variations in the speed of the material gas supplied onto the substrate.
Therefore, the flow paths formed in the cap have been improved to make the speed of the material gas introduced onto the substrate uniform. For example, as for flow paths, in a cap, communicating with a plurality of outlets located on the downstream side (reactor side) of the cap, flow paths joined in a tournament-tree shape toward the upstream side of the cap are adopted, as described in Patent Document 1. Thus, the material gas is distributed while flowing from the upstream side toward the downstream side in the cap, whereby variations in the speed of the material gas supplied from the respective outlets of the cap can be improved. As a result, film thickness distribution of the epitaxial layer grown on the substrate is effectively improved.
However, when a cap having flow paths in a tournament-tree shape as described in Patent Document 1 is manufactured, advanced techniques, such as shaving of a stainless-steel material and diffusion bonding, are required, resulting in an increase in cost. When such a cap is used, another component corresponding to the cap is required. For example, an insert having flow paths as many as the tournament tree-shaped flow paths of the cap is required. In addition, it is necessary to extensively modify the components constituting the passage (the passage having the step portion) communicating with the inside of the reactor into which the material gas is introduced from the insert. Along with this modification, the top dome, which is a component constituting the ceiling of the reactor, needs to be modified, and moreover, a base ring, which is a component to be a base of the reactor, needs to be modified. In addition, when the base ring is modified, man-hours for lifting the base ring up and down are also needed. Therefore, when a cap having tournament tree-shaped flow paths as described in Patent Document 1 is used, much cost and labor are required for manufacture and replacement of necessary components. Some of vapor phase growth apparatuses to be operated do not manufacture a high-quality epitaxial wafer used for advanced device. Therefore, such a cap requiring much cost and labor cannot be easily applied to all vapor phase growth apparatuses.
Meanwhile, in Patent Documents 2 and 3, a quartz member, which is relatively easy to be machined, is used as a member for guiding material gas into a reactor.